Before the amounts and identities of biological agents (e.g., bacteria, fungus, insects, parasites or mammalian cells) in a sample, such as food, soil or a clinical specimen, can be determined, several steps must first be performed.
As an initial step, the sample is usually suspended in a proper buffer, followed by blending or "stomaching" to release the biological agents from the sample. Stomaching was first introduced about twenty years ago [Sharp, A. N. et al., Appl. Microbiol. 24:175-178 (1972] and has since become a useful alternative to blending in the preparation of foods for microbiological analysis.
The biological agents in the sample suspension are thereafter separated from the sample by filtration. The biological agents in the filtrate are most of the time far too diluted, entailing the need of enrichment (i.e., growth of the biological agents) to increase the number of the agents.
Enrichment is a time-consuming process and may take as long as 2 weeks for certain types of biological agents. Also, contamination may occur during the enrichment process. Furthermore, it renders reliable quantification of the biological agents almost impossible.